Low capacitance signal diodes, as exemplified by a standard part number 1N4148, which have a total packaged capacitance of approximately one picofarad (pF) comprise a planar junction structure 50 as illustrated in FIG. 1. In such diodes, the capacitance arises from two primary sources, the first being the PN junction semiconductor capacitance 52 and the second being the capacitance between the semiconductor substrate 65 and the pin 54 as well as pin 56 to pin 54 capacitance. These capacitances are in parallel and thus add to form the total capacitance. To minimize the capacitance of this structure, the area of the PN junction is reduced to typically 20-30 square mils and the spacing between the pins as well as the die and the pins is increased by inserting a plated silver ball 58. The silver ball provides a 2 mil spacing between the semiconductor and the pin while providing a conductive path between the pin 54 and the P type region 60. The resulting structure is surrounded by glass 62 to provide a hermetic seal. Glass 62 may be pushed in to abut the semiconductor 65 in some areas.
The insertion of the silver ball lowers the capacitance but reduces the contact area of the top contact 67 to typically less than 20% of the backside contact 68 and introduces a mechanical strain due to the large mismatch of the coefficient of thermal expansion between the silver ball and the semiconductor as well as the glass and pins. In addition, the silver ball is malleable but not resilient and will work harden and shrink during thermal cycling. As a result, diode failures, either in stress testing or in practice, are almost always associated with the silver ball.
As a result of the problems with the plated silver ball approach, attempts have been made to provide a diode without a silver ball having a complete semiconductor structure between the connecting pins. However, such diodes 70 illustrated in FIG. 2, exemplified by the 1N4148-TXV produced by Microsemi Corporation of California, require an unusually elongated and fragile mesa 72 extending between the connecting pins 73 and 74 to achieve a low capacitance diode. Even with this very fragile structure, the capacitance is still typically twice that of the silver ball type construction (about 2 pF). The reason for the fragile structure and the higher capacitance is that the height provided by the silver ball has to be made up by additional semiconductor material. When mesa semiconductor regions are formed to produce the diode characteristics, the area of the semiconductor contact region 75 must be smaller than the junction area 76. The thicker the N type layer, the bigger the difference between the contact area and the junction area. This means that the N type layer should be kept as thin as possible, increasing the die-to-pin (78-73) and pin-to-pin (74-73). capacitance. Because of strength requirements, the top of the mesa has a minimum dimension which forces the PN junction area 76 to have an even bigger dimension and thus increasing the junction capacitance as well. In addition to higher capacitance, the area of the top contact is now even smaller than with the silver ball approach. However, this large mesa 72 structure does allow the glass 79 to be pushed completely in forming a voidless or cavity free structure. This part is made using the more expensive Tungsten pins and high temperature glass as opposed to the Dumet pins and soft glass of the typical DO-35 packaged 1N4148.
A further problem of conventional PN junction diodes is demonstrated when the diodes are put into reverse breakdown such that they go into thermal runaway. In such circumstances, the diode internal temperature is sufficient to allow destructive alloying to occur. Such destructive alloying is produced by the metallization on the semiconductor material melting and alloying through the junction area. Thus, diodes subjected to thermal runaway produce metallized conductive paths through the junction of the diode, making the diode now permanently conductive. Upon removal of the reverse breakdown condition, the diode fails to recover its previous operating characteristics and is rendered permanently defective.